Because urea is the primary waste carrier of nitrogen for mammals, measurement of dissolved urea is of interest to biomedical, agricultural and environmental professionals. Many techniques for measurement of urea have been developed in the biomedical industry for analyzing biological fluids such as blood or urine so as to monitor renal function and for control of artificial dialysis. For example, U.S. Pat. No. 5,008,078, issued Apr. 16, 1991, inventors Yaginuma et al., describes an analysis element in which gaseous ammonia may be analyzed from liquid samples such as blood, urine, lymph and the like biological fluids. U.S. Pat. No. 5,858,186, issued Jan. 12, 1999, inventor Glass, describes a urea biosensor for hemodialysis monitoring which uses a solid state pH electrode coated with the enzyme urease and is based upon measuring pH change produced by the reaction products of enzyme-catalyzed hydrolysis of urea.
Milk urea is well correlated to urea in the blood and urine, and thus some of the urea measurement techniques used in those fields have been adapted by the dairy industry for measurement of milk urea in order to balance feed rations for optimal nitrogen efficiency. This optimization often leads to considerable savings in feed costs because protein is the most costly feed supplement. In many locations, reduction of nitrogenous waste from the dairy is an even greater consideration than feed costs. Finally, it has been suggested that high systemic urea levels in dairy cows are associated with poor reproductive performance, which is a serious economic concern on dairy farms.
Most existing sensors for urea use the enzyme urease (EC#3.5.1.5) to hydrolyze urea to ammonium and carbonate. Of these, it is most common to measure changes in the ionic composition of the solution with a pH or other ion selective electrode or by using a conductimetric electrode. These delicate electrodes, however, are susceptible to fouling with the high lipid and protein concentration of milk, thus limiting their use without expensive and complicated filtering or dialysis systems. Furthermore, these sensors are all dependent on the sample pH and buffering capacity.
Two calorimetric assays for urea are commonly used. One involves the reaction of urea with diacetyl monoxime in acid solution to give a pink complex, and another involves the reaction of ammonia from hydrolyzed urea with phenol to produce the blue dye indophenol. For the reaction with diacetyl monoxime, the milk must first be dialyzed to eliminate interferences due to peptides and other amide bonded molecules. Phenol and the catalyst for its reaction with ammonia are highly toxic. For these reasons, these assays are not well suited for farm applications.
Near infrared spectrographic instruments have also been used to provide analysis of materials, such as to determine the urea content of milk. For example, U.S. Pat. No. 5,912,730, issued Jun. 15, 1999, inventors Dahm et al. describes a spectrographic analysis instrument that is said to result in more accurate measurements.